Memory system with user configurable density/performance option

ABSTRACT

The memory system has one or more memory dies coupled to a processor or other system controller. Each die has a separate memory array organized into multiple memory blocks. The different memory blocks of each die can be assigned a different memory density by the end user, depending on the desired memory performance and/or memory density. The user configurable density/performance option can be adjusted with special read/write operations or a configuration register having a memory density configuration bit for each memory block.

RELATED APPLICATIONS

This is a continuation of U.S. application Ser. No. 12/434,371, titled “MEMORY SYSTEM WITH USER CONFIGURABLE DENSITY/PERFORMANCE OPTION”, filed May 1, 2009 (allowed), which is a continuation of U.S. application Ser. No. 10/861,645, filed Jun. 4, 2004, now U.S. Pat. No. 7,535,759 issued on May 19, 2009, which are assigned to the assignee of the present invention and the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to memory devices and in particular the present invention relates to non-volatile memory devices.

BACKGROUND OF THE INVENTION

Memory devices are typically provided as internal, semiconductor, integrated circuits in computers or other electronic devices. There are many different types of memory including random-access memory (RAM), read only memory (ROM), dynamic random access memory (DRAM), synchronous dynamic random access memory (SDRAM), and flash memory.

Flash memory devices have developed into a popular source of non-volatile memory for a wide range of electronic applications. Flash memory devices typically use a one-transistor memory cell that allows for high memory densities, high reliability, and low power consumption. Common uses for flash memory include personal computers, personal digital assistants (PDAs), digital cameras, and cellular telephones. Program code and system data such as a basic input/output system (BIOS) are typically stored in flash memory devices for use in personal computer systems.

The present trend of electronic devices is increased performance at reduced cost. The component manufacturers, therefore, must continue to increase the performance of their devices while decreasing the cost to manufacture them.

One way to increase a flash memory device's density while lowering its manufacturing cost is to use multiple level cells (MLC). Such a device stores two logical bits per physical cell. This reduces the overall cost of the memory. NAND flash memory devices are designed to operate in either one of two configurations on the same die: single bit per cell (SBC) or MLC. The selection of the configuration is done at the factory when the die is manufactured through a metal mask or a programmable fuse option.

However, an MLC die, while having improved cost versus density, has drawbacks relative to performance. Both the programming and read operations can become slower for an MLC die. Therefore, the user typically has to choose between having high memory density at low cost and lower memory density with higher performance.

For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a memory system that combines the attributes of both MLC and SBC.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a diagram of one embodiment of a NAND flash memory array of the present invention.

FIG. 2 shows a block diagram of one embodiment of a flash memory system of the present invention that incorporates the memory array of FIG. 1.

FIG. 3 shows a flowchart of one embodiment of a method for determining the desired density/performance configuration of the flash memory system.

FIG. 4 shows a flowchart of one embodiment of a method for configuring the density/performance of the flash memory system.

FIG. 5 shows a flowchart of another embodiment of a method for configuring the density/performance of the flash memory system.

DETAILED DESCRIPTION

In the following detailed description of the invention, reference is made to the accompanying drawings that form a part hereof and in which is shown, by way of illustration, specific embodiments in which the invention may be practiced. In the drawings, like numerals describe substantially similar components throughout the several views. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims and equivalents thereof.

FIG. 1 illustrates a NAND flash array is comprised of an array of floating gate cells 101 arranged in series strings 104, 105. Each of the floating gate cells are coupled drain to source in the series chain 104, 105. Word lines (WL0-WL31) that span across multiple series strings 104, 105 are coupled to the control gates of every floating gate cell in order to control their operation. The memory array is arranged in row and column form such that the word lines (WL0-WL31) form the rows and the bit lines (BL1-BL2) form the columns.

In operation, the word lines (WL0-WL31) select the individual floating gate memory cells in the series chain 104, 105 to be written to or read from and operate the remaining floating gate memory cells in each series string 104, 105 in a pass through mode. Each series string 104, 105 of floating gate memory cells is coupled to a source line 106 by a source select gate 116, 117 and to an individual bit line (BL1-BL2) by a drain select gate 112, 113. The source select gates 116, 117 are controlled by a source select gate control line SG(S) 118 coupled to their control gates. The drain select gates 112, 113 are controlled by a drain select gate control line SG(D) 114.

The memory cells illustrated in FIG. 1 can be operated as either single bit cells (SBC) or multilevel cells (MLC). Multilevel cells greatly increase the density of a flash memory device. Such cells enable storage of multiple bits per memory cell by charging the floating gate of the transistor to different levels. MLC technology takes advantage of the analog nature of a traditional flash cell by assigning a bit pattern to a specific voltage range stored on the cell. This technology permits the storage of two or more bits per cell, depending on the quantity of voltage ranges assigned to the cell.

For example, a cell may be assigned four different voltage ranges of 200 mV for each range. Typically, a dead space or guard band of 0.2V to 0.4V is between each range. If the voltage stored on the cell is within the first range, the cell is storing a 00. If the voltage is within the second range, the cell is storing a 01. This continues for as many ranges are used for the cell.

The embodiments of the present invention may refer to the MLC as a high density configuration. The embodiments of the present invention are not limited to two bits per cell. Some embodiments may store more than two bits per cell, depending on the quantity of different voltage ranges that can be differentiated on the cell. Therefore, the term high density generally refers to any density beyond single bit cells.

FIG. 2 illustrates a block diagram of one embodiment of a flash memory system 220 of the present invention that incorporates the memory array illustrated in FIG. 1. The memory device 200 has been simplified to focus on features of the memory that are helpful in understanding the present invention. A more detailed understanding of internal circuitry and functions of flash memories are known to those skilled in the art.

A processor 210 controls the operation of the flash memory system. The processor 210 may be a microprocessor, a microcontroller, or some other type of control circuitry that generates the memory control, data, and address signals required by the memory device 200.

The memory device 200 includes an array of flash memory cells 230 as discussed previously. An address buffer circuit 240 is provided to latch address signals provided on address input connections A0-Ax 242. Address signals are received and decoded by a row decoder 244 and a column decoder 246 to access the memory array 230. It will be appreciated by those skilled in the art, with the benefit of the present description, that the number of address input connections depends on the density and architecture of the memory array 230. That is, the number of addresses increases with both increased memory cell counts and increased bank and block counts.

The memory device 200 reads data in the memory array 230 by sensing voltage or current changes in the memory array columns using sense amplifier/buffer circuitry 250. The sense amplifier/buffer circuitry, in one embodiment, is coupled to read and latch a row of data from the memory array 230. Data input and output buffer circuitry 260 is included for bi-directional data communication over a plurality of data connections 262 with the controller 210. Write circuitry 255 is provided to write data to the memory array.

Control circuitry 270 decodes signals provided on a control bus 272. These signals are used to control the operations on the memory array 230, including data read, data write, and erase operations. The control circuitry 270 may be a state machine, a sequencer, or some other type of controller. The control circuitry 270, in one embodiment, is responsible for executing the embodiments of the methods of the present invention for configuring the memory blocks as high or single density.

The control circuitry 270 can also program the configuration registers 280 in which, in one embodiment, the high/single density memory configuration bits of the present invention can reside. This register may be a non-volatile, programmable fuse apparatus, a volatile memory array, or both. The configuration register 280 can also hold other data such as trimming data, memory block lock data, record keeping data for the memory device, and other data required for operation of the memory device.

In one embodiment, random access memory (RAM) 290 is included in the system 220 for volatile storage of data. The RAM 290 might be used to store memory density configuration data that is read from the non-volatile memory array 230 during initialization. In an embodiment where the system 220 is a memory card, the RAM 290 might be included on the card or coupled to the card through a back plane or other bus transmission structure.

The flash memory system illustrated in FIG. 2 can be implemented on a memory card, as a computer system, or any other type of non-volatile memory system requiring a system controller and non-volatile memory. Additional embodiments may include more than one memory device coupled to the processor. In such an embodiment, the processor could determine which blocks in the different flash memory devices would be assigned to have information stored in SBC or MLC. Such a system allows the end user to optimize for best performance, cost, and memory density depending on system use.

FIG. 3 illustrates a flowchart of one embodiment of a method for determining the desired density/performance configuration of the memory system of FIG. 2. In one embodiment, the method begins when the system is powered up or otherwise initialized. As part of the initialization process or after the initialization process has been completed, the desired density/performance configurations for the flash memory in the system are obtained 301.

The desired density/performance configurations can be obtained in various ways, depending on the embodiment. A user can input some type of performance parameter that is used by the system to generate the memory block configurations of the present invention.

In one embodiment, the user provides the system with a desired average memory access speed performance parameter and the system determines the memory density required to obtain or come closest to that speed. For example, the processor may have an algorithm or table stored in memory (i.e., RAM or flash) that lists or determines the quantity of memory blocks that should be set to SBC and/or MLC in order to achieve an average access speed. Such an embodiment may require different flash memory dies of the system to have different quantities of memory blocks set to different densities. In the alternative, the user may just input that high access speed is desired. In this case, the system processor can set all of the flash memories to SBC.

In another embodiment, the user may input that a high reliability performance parameter is important. In this case, the processor can set the flash memories to SBC in order to get the highest reliability possible. The processor can also set only certain blocks of each flash memory die or a certain quantity of entire flash memory dies to obtain the desired mean time before failure (MTBF) or other reliability indicator. The processor can access a table stored in non-volatile memory or RAM that indicates the required quantity of memory blocks that are set to SBC and/or MLC in order to achieve a predetermined MTBF or range of MTBFs. An algorithm or other means can also be used to determine the quantity of SBC/MLC blocks are required to achieve certain MTBFs.

In yet another embodiment, the user may input the planned use of the memory system and the system processor determines the best memory density necessary to meet that need. For example, if the memory system is a memory card and the user inputs that the card is to be used to store photographs, reliability is not as important as memory density and a predetermined quantity or all of the flash memory dies may be set to MLC. If the user is to use the memory card to store code, reliability is more important than density and all or a predetermined quantity of the flash memory dies are set to SBC.

After the memory density/performance configurations have been generated 301, the memory blocks of the flash memory die or dies are then configured 305. The configuration can be accomplished in many ways. Two embodiments for configuring the density/performance of flash memory are illustrated in FIGS. 4 and 5.

FIG. 4 illustrates a flowchart of one embodiment of a method for configuring the density/performance of a memory device 305. This embodiment uses special write and read commands to perform high density program and read operations. This embodiment puts the burden on the memory control circuitry to determine the density/performance configuration for a particular block. By having the control circuitry perform this task, the memory device does not require any extra hardware in order to switch blocks between high density and single density. The controller tracks the density/performance level.

This embodiment uses two sets of algorithms—one for SBC reading and writing and another for MLC reading and writing. A higher level routine determines which set of algorithms to use depending on the received command. In this embodiment, the erase operation is substantially similar for each memory density.

The method determines if the received command is a read or write command 401. If a write command was received, it is determined 403 whether the command is a single density write command or a special high density write command. A high density write command 407 causes the controller circuitry to program the specified memory block with two or more bits per cell. A single density write command 409 causes the controller circuitry to program the specified memory block with one bit per cell.

If the received command is a read command, it is determined 405 whether the command is a single density read command or a high density read command. If the command is a high density read command 411, the memory block was previously programmed as an MLC cell and is, therefore, read with a high density read operation. A single density configuration read command causes the memory block to be read 413 assuming it was programmed as an SBC.

FIG. 5 illustrates another embodiment for configuring the density/performance of a memory device 305. In this embodiment, the SBC or MLC configuration is pre-assigned to blocks of memory using a configuration register. This could occur during or after system initialization. This embodiment uses flash memory read and write commands. Additionally, an existing register could be used to store the configuration data so that additional hardware is not required or, in another embodiment, a dedicated configuration register could be added to the memory device.

In one embodiment, the register of the present invention has a bit for every memory block for indicating the operating mode (e.g., MLC or SBC) of that particular block. For example, a logical 1 stored in the memory block 0 configuration bit would indicate that the block is an SBC block while a logical 0 would indicate the block is operating as an MLC block. In another embodiment, these logic levels are reversed.

Alternate embodiments can assign different quantities of blocks to each bit of the configuration register. For example, the register may have a configuration bit assigned to more than one memory block. Additionally, a configuration bit may be assigned to the sub-block level such that each block has multiple configuration bits.

In one embodiment, row 0 of the flash memory device of the present invention is a configuration row. At initialization and/or power-up of the device, the configuration data from row 0 is loaded into the configuration register 501.

When a command is received, it is determined whether it is a read or write command 503. For a read command, the configuration register is checked prior to the read operation to determine if the memory block has been assigned a high density or single density configuration 507. In a single density configuration 511, a single density read operation is performed 519. In a high density configuration 511, a high density read operation is performed 517.

If a write command was received, the configuration register is checked prior to write operation to determine if the memory block has been assigned a high density or a single density configuration 509. In a single density configuration 509, a single density write operation is performed 515. In a high density configuration 509, a high density write operation is performed 513.

In the embodiment of FIG. 5, the user determines the configuration of each block, or other memory cell grouping, and stores this data into the configuration register. When the memory device is powered down, the data in the configuration register is copied to row 0 for more permanent storage in non-volatile memory. In another embodiment, the user can store the configuration directly to the non-volatile, configuration row of the memory device.

The flash memory of the present invention is comprised of memory blocks that can each be configured to store data in different densities. For example, one use of a single memory device might be to store both pictures and code. The picture data is more tolerant of corrupted data than the storage of code. Therefore, since the SBC configuration has a higher reliability than the MLC configuration, the user would typically choose the SBC configuration for the code storage and the MLC configuration for the picture storage.

Similarly, since the MLC configuration might be eight to nine times slower in read and programming performance as compared to the SBC configuration, the user might choose the MLC configuration for memory blocks requiring faster read/write times. This could be useful in a system having fast bus speeds requiring fast storage and retrieval times.

CONCLUSION

In summary, the embodiments of the present invention enable a memory system user to select between an MLC and an SBC configuration. Different configurations can be set up for different memory blocks over multiple flash memory dies in the memory system. Additionally, the density/performance configuration changes can be performed dynamically with configuration commands.

Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement that is calculated to achieve the same purpose may be substituted for the specific embodiments shown. Many adaptations of the invention will be apparent to those of ordinary skill in the art. Accordingly, this application is intended to cover any adaptations or variations of the invention. It is manifestly intended that this invention be limited only by the following claims and equivalents thereof. 

What is claimed is:
 1. An apparatus comprising: a memory array comprising a block of memory cells; control circuitry configured to control operations on the memory array; and a configuration register coupled to the control circuitry and configured to store a density configuration bit, wherein the density configuration bit is assigned to a sub-block of the block of memory cells and wherein the block of memory cells is assigned multiple configuration bits.
 2. The apparatus of claim 1, wherein a memory device includes the memory array, the control circuitry and the configuration register, and wherein the apparatus further comprises a processor coupled to the memory device.
 3. The apparatus of claim 2, further comprising a random access memory (RAM) to store memory density configuration data that is read from the memory array.
 4. The apparatus of claim 1, wherein the configuration register is configured to also store at least one of trimming data, memory block lock data, and record keeping data.
 5. The apparatus of claim 1, wherein the configuration register is further configured to store another density configuration bit, wherein the other density configuration bit is assigned to more than one block of memory cells.
 6. The apparatus of claim 1, wherein the control circuitry is configured to load configuration data from the memory array into the configuration register.
 7. The apparatus of claim 1, wherein the control circuitry is configured to load configuration data from the memory array into the configuration register at initialization of a device including the memory array, the control circuitry and the configuration register.
 8. The apparatus of claim 1, wherein the control circuitry is configured to load configuration data from the memory array into the configuration register at power-tip of a device including the memory array, the control circuitry and the configuration register.
 9. The apparatus of claim 1, wherein the control circuitry is configured to check the configuration register to determine a configuration assignment of the block.
 10. The apparatus of claim 1, wherein the control circuitry is configured to copy data in the configuration register to the memory array in relation to powering down of a device including the memory array, the control circuitry and the configuration register.
 11. An apparatus comprising: a memory array comprising a block of memory cells; control circuitry configured to control operations on the memory array, the control circuitry configured to set memory blocks of the memory to one of a high density configuration or a low density configuration to achieve a desired reliability indicator.
 12. The apparatus of claim 11, wherein a memory device includes the memory array, the control circuitry and the configuration register, and wherein the apparatus further comprises a processor coupled to the memory device.
 13. The apparatus of claim 11, wherein the desired reliability indicator is a mean time before failure.
 14. The apparatus of claim 11, wherein the desired reliability indicator is a range mean times before failure.
 15. The apparatus of claim 11, and further comprising a table stored in the memory, the table configured to contain entries indicating a quantity of memory blocks that are to be set to the high density configuration and the low density configuration in order to achieve the desired reliability indicator.
 16. The apparatus of claim 11, and further comprising a table stored in the memory, the table configured to store density configuration bits for memory blocks of the memory having the high density configuration and the low density configuration.
 17. The apparatus of claim 11, and further comprising a configuration register coupled to the control circuitry, the configuration register configured to store a configuration bit indicating an operating mode for each memory block of the memory.
 18. The apparatus of claim 11, and further comprising a configuration register coupled to the control circuitry, the configuration register configured to store a plurality of configuration bits, and wherein a configuration bit of the configuration register is configured to store density configuration information for more than one block of memory cells of the memory array.
 19. An apparatus comprising: a memory array comprising a block of memory cells; control circuitry configured to control operations on the memory array; and a configuration register coupled to the control circuitry and configured to store density configuration bits to set memory blocks of the memory to one of a high density configuration or a low density configuration to achieve a desired reliability indicator.
 20. The apparatus of claim 19, wherein a memory device includes the memory array, the control circuitry and the configuration register, and wherein the apparatus further comprises a processor coupled to the memory device. 